#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int main()
{
    int n;
    double k, sum = 0;
    scanf("%d", &n);

    for (int i = 0; i < n; i++)
    {
        scanf("%lf", &k);
        sum += 1 / k;
    }

    printf("%.2lf", 1 / (sum / n));
    return 0;
}